1. Field of the Invention
This invention relates to a surface acoustic wave motor using the traveling wave of surface acoustic waves excited on a piezoelectric substrate.
2. Prior Art
An actuator using an electric motor has been used heretofore for driving a photographing lens of a camera, but the disadvantages such as an increase in size of an apparatus, the generation of a magnetic field, the generation of noise and the like have been pointed out. As a means for overcoming the disadvantages, recently an ultrasonic motor has been proposed, which is adapted to take out the mechanical vibration generated by an ultrasonic vibrator mainly through the frictional force and convert the same into the rectilinear motion or the rotary motion. Further as a motor for enabling the precise drive control, it has been proposed that the motor use a traveling wave of surface acoustic waves (See Japanese Patent Laid-Open No. 07-231685, Japanese Patent Laid-Open No. 09-233865).
The constitution and driving principle of a surface acoustic wave motor will now be described with reference to FIGS. 6 and 7. FIG. 6 is a plan view showing the basic configuration of the surface acoustic wave motor, and FIG. 7 is a side view thereof.
In FIGS. 6 and 7, a surface acoustic wave motor 100 is so constructed that a comb-shaped electrode having interdigital structure 102 is disposed on a piezoelectric substrate 101 which is a substrate formed of piezoelectric material such as piezoelectric ceramic material mainly composed of PZT (PbZrO.sub.3.PbTiO.sub.3), and connected to a high frequency power supply 103.
Vibration absorbers 107, 108 are arranged at the ends of the piezoelectric substrate 101. These are intended to absorb surface acoustic wave vibration reaching the ends of the piezoelectric substrate 101 so that a standing wave is not generated in the piezoelectric substrate.
When the comb-shaped electrode having interdigital structure 102 is excited by the high frequency power supply 103, surface acoustic waves (Rayleigh waves) L1, L2 vibrating backwardly elliptically are generated on the right and on the left of the comb-shaped electrode having interdigital structure 102 in the piezoelectric substrate 101, and respectively travel in the direction of going away from the comb-shaped electrode having interdigital structure 102. That is, the surface acoustic wave L1 travels in the direction of an arrow (a), and the surface acoustic wave L2 travels in the direction of an arrow (b) (the opposite direction to the arrow (a)).
A solid state slider 109 placed on the piezoelectric substrate 101 gets on the crest of the surface acoustic wave L1 or L2 vibrating backwardly elliptically, so that it is moved in the direction of approaching the comb-shaped electrode having interdigital structure 102 which is the opposite direction to the traveling directions of the surface acoustic waves L1 and L2. That is, as shown in FIG. 7, when the slider 109 gets on the crest of the surface acoustic wave L1, it is moved in the direction of an arrow (C).
When the slider 109 reaches a position striding over the comb-shaped electrode having interdigital structure 102, the slider 109 gets on the crests of the surface acoustic waves L1 and L2 traveling in the opposite directions to each other so that the slider cannot be moved in either direction. Accordingly, in the configurations shown in FIGS. 6 and 7, the slider 109 is capable of moving in only one direction.
For application to a general driving mechanism such as the movement of a lens of a camera, it is necessary to be able move on one axis in both directions. Therefore, it has been proposed to construct a surface acoustic wave motor adapted to move a slider in a designated direction by arranging two comb-shaped electrodes having interdigital structure on a piezoelectric substrate and driving one of the comb-shaped electrodes having interdigital structure.
FIG. 8 is a perspective view showing the basic construction of a surface acoustic wave motor in which two comb-shaped electrodes having interdigital structure are arranged on a piezoelectric substrate, and FIG. 9 is its plan view.
In FIGS. 8 and 9, a surface acoustic wave motor 120 is so constructed that a first comb-shaped electrode having interdigital structure 102 and a second comb-shaped electrode having interdigital structure 104 are arranged on a piezoelectric substrate 101 and respectively connected to a first high frequency power supply 103 and a second high frequency power supply source 105. A slider 109 is arranged between the first comb-shaped electrode having interdigital structure 102 and the second comb-shaped electrode having interdigital structure 104. Vibration absorbers 107, 108 are arranged at the ends of the piezoelectric substrate 101.
In this arrangement, in the case of moving the slider 109 in the direction of an arrow (d) (See FIGS. 8 and 9), the first comb-shaped electrode having interdigital structure 102 is excited by the high frequency power supply 103 to generate a surface acoustic wave propagated to the left (in the opposite direction to the arrow (d)). Thus, the slider 109 can be moved toward the first comb-shaped electrode having interdigital structure 102 (in the direction of an arrow (d)).
In the case of moving the slider 109 in the opposite direction of the arrow (d), the second comb-shaped electrode having interdigital structure 104 is excited by the high frequency power supply 105 to generate a surface acoustic wave propagated to the right in FIGS. 8 and 9, thereby achieving the movement.
Though the thus constructed surface acoustic wave motor has high driving speed and is excellent in responsiveness, the energy efficiency is very low. This is because most of the surface acoustic wave energy is not used for moving the slider, but is absorbed in the ends of the piezoelectric substrate.
That is, since in the thus constructed surface acoustic wave motor, vibration absorbers are disposed at the ends of the piezoelectric substrate so as not to generate a standing wave in the piezoelectric substrate, most of surface acoustic wave energy generated on the piezoelectric substrate is absorbed in the vibration absorbers, resulting in the disadvantages that the generation of heat is large so that continuous driving is difficult and very large driving power is needed.
As a countermeasure, an energy recovery type surface acoustic wave motor has been proposed, which is so constructed that the surface acoustic wave energy generated in the piezoelectric substrate to reach the ends thereof is not absorbed in the vibration absorbers at the ends of the piezoelectric substrate, but the energy is recovered to be circulated (See Japanese Patent Laid-Open No. 11-146665).
FIG. 10 is a plan view for explaining an example of construction of an energy recovery type surface acoustic wave motor 200, in which a first comb-shaped electrode having interdigital structure 202 and a second comb-shaped electrode having interdigital structure 203 for generating surface acoustic waves are disposed at a spacing of 1/4 .lambda. of the wavelength .lambda. of the generated surface acoustic wave on a piezoelectric substrate 201, and respectively connected to a first high frequency power supply 204 and a second high frequency power supply 205.
In addition to the above, a third comb-shaped electrode having interdigital structure 206 and a fourth comb-shaped electrode having interdigital structure 207 which are provided with an inductance for recovering surface acoustic wave energy and re-exciting the surface acoustic wave are disposed on the piezoelectric substrate 201.
The piezoelectric substrate and the third comb-shaped electrode having interdigital structure 206 and the fourth comb-shaped electrode having interdigital structure 207 disposed thereon constitute an electromechanical transducer element, which functions as a mechanical-electric transducer element for converting the mechanical vibration into the high frequency electric power when the surface acoustic wave propagated on the piezoelectric substrate is received, and also functions as an electromechanical transducer element for converting the high frequency electric power into the surface acoustic wave power which is mechanical vibration when the high frequency electric power is input.
An inductance 208 is connected in parallel to the third comb-shaped electrode having interdigital structure 206, and an inductance 209 is connected in parallel to the fourth comb-shaped electrode having interdigital structure 207. These inductances are provided for restraining reflection of the surface acoustic waves propagated on the piezoelectric substrate and re-exciting the same.
A slider 210 is disposed between the first comb-shaped electrode having interdigital structure 202 and the fourth comb-shaped electrode having interdigital structure 207.
In this arrangement, the phase of high frequency voltage applied to the first comb-shaped electrode having interdigital structure 202 and the second comb-shaped electrode having interdigital structure 203 is shifted to control the traveling direction of generated surface elastic waves.
In this arrangement, at the time of moving the slider 210 to the right (in the direction of an arrow (e) in FIG. 10, it will be sufficient to generate the surface elastic wave toward the left (in the opposite direction to the arrow (e))
First, voltage, EQU V1=V01.multidot.sin(.omega.t),
is applied from the high frequency power supply 204 to the first comb-shaped electrode having interdigital structure 202, and voltage EQU V2=V02.multidot.sin(.omega.t-.pi./2),
is applied from the high frequency power supply 205 to the second comb-shaped electrode having interdigital structure 203 to drive the second comb-shaped electrode having interdigital structure 203 with a phase difference of .pi./2 to the first comb-shaped electrode having interdigital structure 202.
On the piezoelectric substrate 201, surface acoustic waves heading toward the left (in the opposite direction to the arrow (e)) in FIG. 10 are generated, and the surface acoustic waves propagated on the piezoelectric substrate 201 are converted into the high frequency electric power by the fourth comb-shaped electrode having interdigital structure 207. The converted high frequency electric power is circulated and applied to the third comb-shaped electrode having interdigital structure 206, and again converted to the surface acoustic waves heading toward the left (in the opposite direction to the arrow (e)) to excite the piezoelectric substrate 201. Thus, the slider 210 can be moved toward the right (in the direction of the arrow (e)) in FIG. 10.
At the time of moving the slider 210 to the left (in the opposite direction to the arrow (e)) in FIG. 10, it will be sufficient to generate the surface acoustic waves toward the right (in the direction of the arrow (e)) in FIG. 10.
First, voltage, EQU V1=V01.multidot.sin(.omega.t-.pi./2),
is applied from the high frequency power supply 204 to the first comb-shaped electrode having interdigital structure 202, and voltage, EQU V2=V02.multidot.sin(.omega.t),
is applied from the high frequency power supply 205 to the second comb-shaped electrode having interdigital structure 203 to drive the electrode.
On the piezoelectric substrate 201, surface acoustic waves heading toward the right (in the direction of the arrow (e)) in FIG. 10 are generated, and the surface acoustic waves propagated on the piezoelectric substrate 201 are converted into the high frequency electric vibration by the third comb-shaped electrode having interdigital structure 206. The converted high frequency electric vibration is circulated and applied to the fourth comb-shaped electrode having interdigital structure 207, and again converted into the surface acoustic waves heading toward the right (in the direction of the arrow (e)) to excite the piezoelectric substrate 201. Thus, the slider 210 can be moved toward the left (in the opposite direction to the arrow (e)) in FIG. 10.
Thus, the surface acoustic waves propagated on the piezoelectric substrate to one end thereof are recovered by the comb-shaped electrode having interdigital structure disposed at one end of the piezoelectric substrate, and circulated to the comb-shaped electrode having interdigital structure disposed at the other end of the piezoelectric substrate to re-excite the piezoelectric substrate, so that the energy efficiency can be heightened.
In the energy recovery-type surface acoustic wave motor, in addition to the ordinary surface acoustic wave generating comb-shaped electrode having interdigital structure, there are provided two comb-shaped electrodes having interdigital structure to which an inductance is connected in parallel, whereby the surface acoustic waves propagated on the piezoelectric substrate to the end are recovered by one comb-shaped electrode having interdigital structure, and circulated to the other comb-shaped electrode having interdigital structure. This arrangement, however, has the disadvantages that even if the optimum value of inductance is selected, actually it is difficult to hold down the reflection of surface acoustic waves at the ends of the piezoelectric substrate, and further when the recovered surface acoustic wave energy is again emitted from the other comb-shaped electrode having interdigital structure, the energy is emitted from both ends of the comb-shaped electrode having interdigital structure so that an energy loss is caused, and the improvement in energy efficiency has its limit.